This section provides background information related to the present disclosure which is not necessarily prior art.
Over the past two decades, photonic crystals have enabled studies of many interesting physical phenomena and have been increasingly used in applications. These periodic structures possess band structures that can be exploited to engineer the electromagnetic response of a given system, and are analogous to the electronic bands of crystalline materials.
A specific class of photonic crystals have their periodicity confined to a thin layer that is surrounded by a low-index material; consequently they have bands that extend into the light cone. These leaky modes can be excited by incident plane waves to produce Fano line shapes and similarly decay into the continuum when the excitation source is removed. The lifetime of an excited mode and its associated coupling strength to the continuum is largely determined by the mutual symmetry of the mode and permissible outgoing waves. Select modes possess infinite lifetimes at zone center as a result of their symmetry mismatch with allowed radiation modes.
These symmetry-protected modes have recently been used to demonstrate high-quality factor resonances near normal incidence that may be exploited for various applications. Particular one-dimensional photonic crystal slabs, often called high-contrast gratings, have demonstrated spectral engineering capabilities including ultra-broadband reflectors, two-dimensional lenses, and filters.